Trillium with four leaves, and the associated flower parts in 4's, are not that unusual. They turn up fairly regularly among many, if not all species in the genus ( I have seen them in T. decumbens, T. cuneatum, T. luteum, T. simile, T. reliquum, and T. simile that come to mind mind immediately). I have even come across a 5-leaf speciem in the woods.